A sign on the Brattleboro common warns of a coming detour of nearby Route 30 this Wednesday and Thursday.

BRATTLEBORO — Route 30 drivers will face a major detour at the road’s southeastern starting point this week.

Crews are set to divert traffic onto side streets Wednesday and Thursday as they work to remove two 1,000-foot Interstate 91 bridges overhead.

The Vermont Agency of Transportation is spending $59.5 million (the federal government is paying 90 percent and the state the remaining 10 percent) to replace four bridges between Interstate exits 2 and 3.

Interstate motorists driving north or south through Brattleboro have to slow down with the closing of passing lanes during construction, which is set to continue through the summer of 2016.

Underneath the two largest bridges, Route 30 travelers have found the speed limit reduced to 40 mph, with workers periodically limiting traffic there to a single lane.

Contractors are scheduled to close the road below the spans starting at 8 a.m. Wednesday and reopen it at 8 a.m. Friday.

The state also expects construction to periodically slow or stop traffic on the nearby Upper Dummerston Road, West River hiking and biking trail and parts of the waterway itself.

Workers broke ground on Interstate 91 at Vermont’s southern border in 1957 and finished the last link — a stretch from St. Johnsbury to the New Hampshire border — in 1982. Since then, traffic and road salt have especially taken their toll in Brattleboro, whose highway link is the oldest and most outdated.

Current crews expect to work day and night for the next several months until they completely remove the old bridges.